fix(A.3 #25): guard sample_index_for_seconds against NaN and infinity

Review feedback (MINOR): the original <= 0.0 clamp caught negatives
and zero but not non-finite inputs. Rust's saturating float-to-int
cast turns f64::INFINITY into u64::MAX, which would park the capture
buffer origin beyond any reachable sample index and trim the whole
buffer forever if a future end_secs source ever produces infinity
(clock glitch, overflow upstream, corrupted timestamp in a pass).

Adds is_finite() check. NaN, +infinity, -infinity, and zero all
return 0, which downstream trim_buffer_to_commit_point treats as
no-op. Test covers all three non-finite cases.

crates/transcription/src/streaming/buffer_trim.rs

@@ -14,8 +14,14 @@
 /// Absolute sample index at the end of the given session-relative
 /// seconds mark, rounded to the nearest sample. `end_secs` typically
 /// comes from `LocalAgreement::last_committed_end_secs()`.
+///
+/// Guards against non-finite inputs: NaN and ±infinity both return 0
+/// ("nothing committed yet"). Without this, Rust's saturating
+/// float-to-int cast turns `f64::INFINITY` into `u64::MAX`, which
+/// would park the capture buffer origin at an index beyond any
+/// reachable sample and trim the entire buffer forever.
 pub fn sample_index_for_seconds(end_secs: f64, sample_rate: u32) -> u64 {
-    if end_secs <= 0.0 {
+    if !end_secs.is_finite() || end_secs <= 0.0 {
         return 0;
     }
     (end_secs * sample_rate as f64).round() as u64
@@ -65,6 +71,17 @@ mod tests {
         assert_eq!(sample_index_for_seconds(-1.0, 16_000), 0);
     }
 
+    #[test]
+    fn sample_index_for_seconds_rejects_nan_and_infinity() {
+        // Defensive against non-finite inputs: without the is_finite()
+        // check, Rust's saturating float-to-int cast makes +infinity
+        // become u64::MAX, which would park the buffer origin beyond
+        // reach and trim the whole buffer forever.
+        assert_eq!(sample_index_for_seconds(f64::NAN, 16_000), 0);
+        assert_eq!(sample_index_for_seconds(f64::INFINITY, 16_000), 0);
+        assert_eq!(sample_index_for_seconds(f64::NEG_INFINITY, 16_000), 0);
+    }
+
     #[test]
     fn sample_index_for_seconds_rounds_nearest() {
         // 0.5 s at 16 kHz = 8000 samples exactly.